An optimum moisture level has to be maintained, as lower
moisture level tends to reduce microbial growth, enzyme
stability and substrate swelling [18,19]. Higher moisture level
leads to particle agglomeration, gas transfer limitation and
competition from bacteria [20]. In general, the moisture levels
in SSF processes vary between 30 and 85%. For bacteria, the
moisture of the solid matrix must be higher than 70%, and in
the case of filamentous fungi it could be as wide as 20–70%[21].
The cell concentration and particle size were 4.0× 106 cells/
g raw sorghum and 0.5–1.5 mm, respectively. No reducing
agent was added. Chopped sorghum was dried to the moisture
levels of 60, 65, 70, 75 and 80, respectively. Another 100 g
chopped sorghum stalks were adjusted to reach the moisture
content of 85% by adding distilled water. After sterilization,
each flask was inoculated with concentrated yeast cells. Theflasks were incubated in a constant environment shaker at
37 °C. Samples were withdrawn every six or 4 h.
The moisture level of 75–80% was optimal. It resulted in the
maximum ethanol yield of 7.5 g ethanol per 100 g fresh stalks